Are there FRONT and REAR brake shoes?

[FSJGuy]

See the subject. Are there front and rear brake shoes? Are the front and rear axle the same? They look the same to me. I finally got around to removing the drum which was a breeze thanks to Alan Bennet who previously serviced the brakes and remembered to put ANTI-SEIZE on those pesky 3 flat head screws. ALAn, I LOVE YOU!!! <g>

The shoes LOOK to be the same, but the wear pattern is diffeent on them. The rear shoe was more worn than the front. Maybe this is normal. I haven't worked with drum brakes much.

[LuckyPabst]

The shoe sets are the same front and rear. But at each wheel you have one shoe with short lining and one shoe with long lining. Short lining goes forward always.

Chris

[FSJGuy]

This is interesting, as my shoes look exactly the same. i've only pulled one wheel, so maybe the next one will be different... i've got the part #'s from the online manuals, so I'll see what's available.

[bump_r]

In drum brakes, there IS a primary and secondary shoe. Due to the dynamic rotational forces (drum) versus the resistance (the anchor pin), one shoe naturally and inescapably does more work. In use the front and rear shoes act more in series than parallel. Pictiure a shoe set-up - driver side, moving to the left - drum is rotating counter-clockwise (Anit-clockwise to our British buds). The wheel cylinder pistons go outward, the shoes move outward - radially - until they contact the drum. At this point, the shoes are imparted with a rotational force. The forward (primary) shoe is pulled away from the anchor pin at the top, and the bottom is driven towards the rear shoe (secondary) via the adjuster. The rear shoe is in turn driven into the anchor pin, where it meets its, well, anchor. Since the wheel cylinder pistons are free floating, they don't bias shoe pressure - they are statically equalized. Introduce the rotational foces, and the dynamic distribution favors the froward shoe. Thus, the forward shoe shoe is called the primary shoe in that it drives the rear shoe, not because it works harder.

An interesting note is that as the rear shoe is bottomed against the anchor pin, the corresponding piston is driven inward, and in turn forces the froward piston forward - thus the free-floating nature of the wheel cylinder. Picture the rear shoe hinged to the anchor instesd oa it being able to mve freely in and out from it, nad mentally cycle the shoes in and out - you may be able to picture the "series vs parallel" notion above.

I think.

[FSJGuy]

Yup. Your explanation makes sense. Thank you. This explains why the short lining goes forward. I'll also have to check, maybe the lining lengths ARE different, and the metal parts are just the same. I didn't notice.

This also explains why my rear shoe was more worn than the front. Thanks. I have limited funding for this project (duh, don't we all?) so I'm doing it on step at a time. Nice thing is, that it's not a daily driver, so I can have it "down" indefinitely. Maybe next weekend (Easter) I can pull off another wheel. Even though it's just little steps, it's nice to be able to be doing something, vs just letting it sit there.

BTW, Jon (our trusty site admin) is correct in that the 3 screws align the brake drum to the hub. It looks like there is no other alignment surface for the drum to mate to.

Oh, does anyone know the MAX diameter we can turn the drums to? Mine's a little scored from mud and debris. Or should I leave it alone since someone said that drums are hard to find?

Thanks guys!

[Nuke_spicergear]

It's probably the first time they've been turned so you'll prolly be okay. Consider yourself lucky if the guy has a big enough die to hold it.

[brute4c]

I have searched for that spec...never found one for the M715's...even tried Memphis Equipments guys thinking since they redo them maybe they would have a spec from somewhere...they said they didnt and that as long as they werent thin, go for it....now what the heck do we want to call thin....
I do have somewhere, a spec for the civvy trucks 13 inch drums...but I cannot confirm that those drums and our drums are similiar/the same so that we can use that figure....somebody has a dually with 13 inch drums, civvy truck...who was that??
brute4c

[bump_r]

The spec is usually cast into the drom somewhere - maybe not in this case being mil-only item, though.

As far as the three screwd aligning the drums, I'm on that list. I can't recall a pilot bore on the hub that would otherwise do the aligning (some swear it's tehre - maybe a running change in production?)

Anyway - the machining cost to countersink the holes, cost of special screws, and teh general PITA factor leans more to them being needed, not just a "convenient"way to retain the drums before the wheels are attached. Remember these trucks are "lowest bidder" items, so every cost-cutting measure wouldhave been taken that could be thought of. The layout, machining, and necessary quality-control for these dang holes just seems excessive unless htey were mandatory fro drum alignment.

So there.

[Barrman]

If you have trouble getting a brake shop to turn the drums because they are to big to fit their machine. Take them a clean hub and six lug nuts. They can mount the hub on their brake lathe with regular sized collets. Then bolt the drums to the hub.

[brute4c]

Amen Bob...my sentiments exactly...or is that sediments...I guess I'm getting a little to sedentary lately...any, I fully agree...its a lot of work for "fun".
I did look all over my drums for any semblance of a stamping for the max. dia. but never found one.
It would be nice to figure it out...is there a way to 'establish' a safe minimum based on what can be discerned from drums from other manufactures of basically the same materials and load handling? Like if there is a 12 inch that can get down to xx.xxx which leaves a thickness of 0.yyy of material for the shoes to wear on, can we say, ratioing against that number, that our drums should be safe if the max. dia. does not exceed zz.zzz??
Bob, you da man with the closest chance of working this one out I know of...theres a cold one in it for ya at the FE if ya do!!
brute4c